Effect of Hemlock Woolly Adelgid (HWA) Infestation on Water Relations of Carolina and Eastern Hemlock

The Effect of Hemlock Woolly Adelgid (HWA) Infestation on Water Relations of Carolina and Eastern Hemlock: Can Ecophysiological Investigation of Tree Water Relations Improve Silvicultural Management of the HWA?

In eastern North America, hemlock woolly adelgid (HWA; Adelges tsugae Annand) is an exotic insect pest from Asia that is rapidly decimating native eastern hemlock (Tsuga canadensis (L.) Carr.) and Carolina hemlock (Tsuga caroliniana Engelm.). Extensive research has been committed to the ecological impacts and potential control measures of HWA, but the exact physiological mechanisms that cause tree decline and mortality are not known. In Fraser fir (Abies fraseri (Pursh.) Poir.), it is known that balsam woolly adelgid (BWA; Adelges picea Ratz.) produces abnormal xylem in response to infestation (Arthur and Hain 1985) and hemlock trees may be reacting to infestation in a similar manner. We hypothesize that this abnormal xylem obstructs water movement within the trees, causing hemlock trees to die of water-stress.

In this study, water relations within 15 eastern and Carolina hemlock trees were evaluated to determine if infestation by HWA was causing water stress. Water potential, carbon-13 isotope ratio, stem conductivity, and stomatal conductance measurements were conducted on samples derived from those trees. In addition, branch samples were analyzed for possible wood anatomy alterations as a result of infestation (Walker-Lane 2009). Wood anatomy of the branches provided evidence that infested hemlocks are indeed experiencing abnormal wood production in the xylem, which led to reduced water transport capacity in a similar way as in Fraser fir (Hollingsworth et al. 1991). In addition, pre-dawn branch water potential measurements were more negative in infested trees than in non-infested trees. These results indicate that infested eastern and Carolina hemlock are experiencing drought-like symptoms (Arthur and Hain 1986), and that plant uptake of soil water by roots is somehow compromised. However, those symptoms are not due to soil moisture limitations but presumably to a hydraulic decoupling at the soil-root interface. Moreover, carbon isotope ratios of the branches were more positive for infested trees, while stomatal conductance was lower in infested trees. This indicates that the significant decrease in reduced conducting sapwood area, terminal branch growth and leaf area in infested trees were sufficient to influence sap flux and whole-tree water use. A soil-plant water transport model (Sperry et al. 1998) showed that even small summer drought would induce total loss of water transport capacity and would induce irreversible hydraulic failure of the conducting tissue, causing tree death.

We conclude that changes in xylem anatomy and leaf area in response to HWA decreased the capacity for water uptake and transport, placing infested trees at a compounding competitive disadvantage (relative to surrounding trees) to assimilate water and carbon, and acquire soil resources. These physiological responses reduced tree capacity to resist further infestation, eventually leading to tree mortality. However, it is not clear how these physiological stressors interact with environmental conditions (e.g., drought, high irradiance, high evaporative demand) to cause tree death. It has been reported that infested trees die more quickly in warmer and drier locations (Ford and Vose 2007). Our study is consistent with these reports showing a link between HWA and accelerated mortality from drought stress. We predict that HWA will cause higher mortality in the Southeast in warmer and drier locations, which will be aggravated by periodic droughts that further stress infested trees. Projected increase in drought frequency due to climate change is then also predicted to increase HWA mortality. In order to manage HWA and reduce its impact to hemlock resources we propose that forest resource managers plant trees in cooler and wetter locations and if possible on soil with high moisture capacity and the greatest amount of plant available water such as silt loam soil.

Author backgrounds:

1Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, NC 27695